The present invention relates to a high-strength extra fine metal wire with high tenacity to be used for rubber reinforcement of tire cord, belt cord, etc., plastic reinforcement, material for electromagnetic wave shield, needle material, wire saw, precision spring, wire rope, miniature rope, angling thread, etc.
Generally, extra fine metal wire is used in various modes depending on its uses: by twisting a plural number of pieces together, by weaving, in the state of single wire, by cutting in short pieces, etc.
By the way, the properties requested of extra fine metal wire are the possibility of finishing in an extra fine diameter, sufficiently high strength and toughness for resisting uses in the said fields of utilization, excellent workability in drawing and twisting, reasonable cost, etc.
However, conventional extra fine metal wires are usually manufactured through several times of cold wire drawing while preventing a drop of tenacity of wire rod in each wire drawing by submitting hot rolled material (high carbon steel wire rod generally) to several times of patenting during the processing.
For that reason, a lot of manufacturing processes were required with the prior art and the manufacturing cost was rather expensive. Moreover, patenting of extra fine metal wire is technically difficult because of the difficulty of temperature control, and the drawing strain was also limited because of wire breaking, etc. Furthermore, the true strain in the said cold wire drawing was 2.30-3.50 or so at the maximum (true strain .epsilon.=21n Do/Df, Do: Wire diameter before wire drawing, Df: Finished wire diameter) and the finished extra fine metal wire usually had a strength of 300 kgf/mm.sup.2 or under in tensile strength and a wire diameter of 0.15 mm or over.
A steel wire having a tempered martensite structure submitted to hardening and tempering by heat treatment is also known to the public. This steel wire is submitted to wire drawing, etc. by reducing the strength with tempering because it is a wire rod of a comparatively large diameter and cannot provide a good drawability in the hardened state. However, it is rarely utilized in the aid fields of service because it is not an extra fine metal wire of high strength.
In addition, steel wire of a diameter of 1 mm or over having a proper level of strength and tenacity in the tempered state as oil-tempered wire is also used in a large quantity. This steel wire is prepared by oil tempering, etc. because it is poor in tenacity although it has excellent hardness and strength.
Namely, with the prior art, there were such problems that a hardened steel wire is fragile and poor in tenacity and a steel wire submitted to hardening and tempering has an improved tenacity but is difficult to control on heat treatment and its strength may sharply drop depending on the way of tempering. For that reason, wire drawing of a wire rod of patenting structure has so far been considered as the best way for obtaining a high-strength fine metal wire of excellent tenacity and best drawability from a high carbon steel wire rod.
These days, however, with the progress of technology, it has become difficult to sufficiently meet the required quality with an extra fine wire obtained by this wire drawing with patenting, and there is now a request for a high-strength extra fine metal wire of good productivity which not only is rectangular in wire diameter but also maintains a high strength and a high tenacity and is suitable for wire drawing.
By the way, a hardened steel wire has a martensite structure and can hardly be submitted to cold working. Moreover, a steel wire of large diameter is known to improve in strength and proof stress. However, this steel wire has poor drawability with a true strain of 0.69 or so and its tenacity is also not so high with a tensile strength of 250 kgf/mm.sup.2 or so. This is probably because of an influence of its metal structure or roughing of carbide and dispersion in size, etc., according to the observation of this inventor.
Moreover, it is patenting which has so far been considered as the best means of obtaining a high-strength fine metal wire thanks to good drawability. It is a well known fact that this pearlite structure by patenting is a lamellar structure of ferrite and cementite. And its drawability has been believed to be excellent because this cementite is lamellar. Indeed, an extra fine wire of pearlite structure is submitted to wire drawing with a true strain of 3.3 or so. However, the said cementite is about flat in shape as it appears in the micrographic structure and its cross section is rectangular in shape. For that reason, if you make a wire drawing of a higher drawing strain, the wire cracks with interference among its cementite layers, producing breaking, etc. (drawing limit). The drawing limit is about 3.5 in true strain at the best. At a higher drawing strain, breaking of wire often takes place during the wire drawing and the tenacity also suddenly drops, making it impossible to further improve its strength.
The present invention aims at sharply improving the drawability of a wire of a certain chemical composition as well as the strength and tenacity in the state of extra fine metal wire by performing quenching and tempering properly and by controlling the metal structure of the extra fine metal wire obtained by wire drawing at a constant level.
With the high-strength extra fine metal wire of the present invention, it has become possible to perform wire drawing with a true strain of 4.0-4.7 or so and to sharply improve the strength and tenacity of the wire by eliminating interference among the carbides appearing in the micrographic structure thanks to adoption of an about rectangular or circular shape in which the shape of the section is restricted.
Moreover, the high-strength extra fine metal wire of the present invention has a wide variety of applications and a high value of utilization because it has high strength, high tenacity and excellent fatigue resistance which could never be obtained with any conventional metal wire, although it is made of a conventionally used carbon steel wire rod. Moreover, excellent drawability makes it possible to secure a high degree of processing and to also reduce the number of dies in the heat treatment process or wire drawing during the working. The effects of this invention are really remarkable.